Polytetramethylene ether glycol (PTMEG) is a commodity in the chemical industry, widely used in the manufacture of polyurethanes and polyesters. It is commonly prepared by reacting tetrahydrofuran (THF) with a strong acid catalyst, such as fluorosulfonic acid, and then quenching the product with water.
While this process has proved to be quite satisfactory, it is not as efficient as desired because the acid catalyst cannot be recovered and reused. Moreover, disposal of the spent acid catalyst is a problem because of its toxicity and corrosiveness.
A more recent process, as disclosed in U.S. Pat. No. 4,120,903, polymerizes tetrahydrofuran using a polymer containing alpha-fluorosulfonic acid groups and a chain terminator which is water or 1,4-butanol diol. The nature of the catalyst permits its reuse, thereby eliminating the disposal problems and the catalyst's lack of solubility in the reaction mass makes it easy to separate the catalyst from the product at the end of the polymerization reaction. This very low solubility also minimizes loss of catalyst as the reaction proceeds. However, this process produces a polytetramethylene ether glycol having a molecular weight of 10,000 or more, while the commercial products generally have molecular weights of less than 4,000 with the majority of commercial products having a number average molecular weight of either 1,000 or 2,000.
An even more recent U.S. Pat. No. 4,163,115 disclosed that the molecular weight of the polytetramethylene ether glycol product, when using a polymer having alpha-fluorosulfonic acid groups as catalyst can be controlled by adding acylium ion precursor to the reaction medium. The acylium ion precursors were acyl halides and anhydrides of carboxylic acids whose carboxylic acid moieties contain 1 to 36 carbon atoms and especially those of 1 to 4 carbon atoms. Acetic anhydride, propionic anhydride and formic-acetic anhydride were specifically illustrated. The use of mixtures of acetic acid and acetic anhydride as molecular weight control agent is also disclosed.